NUCELLAR PROJECTION TRANSFER CELLS IN THE DEVELOPING WHEAT-GRAIN

Transfer cells in the nucellar projection of wheat grains at 25 +/- 3 days after anthesis have been examined using light and electron micros copy. Within the nucellar tissue, a sequential increase in non-polariz ed wall ingrowth differentiation and cytoplasmic density was evident. Cells located near the pigment strand were the least differentiated. T he degree of differentiation increased progressively in cells further removed from the pigment strand and the cells bordering the endosperm cavity had degenerated. Four stages of transfer cell development were identified at the light microscope level. Wall ingrowth differentiatio n followed a sequence from a papillate form through increased branchin g (antler-shaped ingrowths) which ultimately anastomosed to form a com plex labyrinth. The final stage of wall ingrowth differentiation was c ompression which resulted in massive ingrowths. In parallel with wall ingrowth deposition cytoplasmic density increased. During wall deposit ion, paramural and multivesicular bodies were prominent and were in cl ose association with the wall ingrowths. The degeneration phase involv ed infilling of cytoplasmic islets within the wall ingrowths. This was accompanied by complete loss of the protoplast. The significance of t his transfer cell development for sucrose efflux to the endosperm cavi ty was assessed by computing potential sucrose fluxes across the plasm a membrane surface areas of the nucellar projection cells. Transfer ce ll development amplified the total plasma membrane surface area by 22 fold. The potential sucrose flux, when compared with maximal rates of facilitated membrane transport of sugars, indicated spare capacity for sucrose efflux to the endosperm cavity. Indeed, when the total flux w as partitioned between the nucellar projection cells at the three stag es of transfer cell development, the fully differentiated stage III ce lls located proximally to the endosperm cavity alone exhibited spare t ransport capacity. Stage II cells could accommodate the total rate of sucrose transfer, but stage I cells could nor. It is concluded that th e nucellar projection tissue of wheat provides a unique opportunity to study transfer cell development and the functional role of these cell s in supporting sucrose transport.